1. Field of the Invention
The invention concerns a process for dyeing textile material having polyester fibers.
2. Background of the Invention
In a typical form of process for dying textile material including polyester fibers, the textile material is treated in an alkaline dye bath containing a dispersion dyestuff, at a temperature above about 100.degree. C. In that respect it will be noted that polyester dyeing processes involving a temperature above about 100.degree. C., which are therefore implemented under an increased pressure, make it possible to achieve shorter dyeing times than when operation is implemented under normal pressure. In addition, the quality of dyeing penetration, the compensatory effect in respect of fixing and stretching differences, the dyeing yield and the degrees of color fastness are improved under high-temperature (HT) conditions. However, under HT conditions, the dyeing processes which are usually employed, involving an acid dye bath, suffer from the following problem: during the dyeing procedure, low-molecular oligomers are liberated from the interior of the fibers, and they occur as by-products in the operation for spinning endless polyester fibers. Those low-molecular oligomers are deposited in the acid dye bath upon cooling of the dye liquor on the fiber surface and at the surfaces of the dyeing equipment. Those deposits considerably impair the technical properties of the textile material such as for example the running properties and the feel thereof. In addition the dyeing equipment is soiled and contaminated by the oligomer deposits. Furthermore reductive or alkaline cleaning procedures are required when dealing with dark color shades, when using an acid dye bath.
For that reason the use of alkaline dye baths has been proposed. The solubility of the oligomers is enhanced when operating under alkaline conditions. Furthermore, operating under alkaline conditions provides for partial saponification of the liberated oligomers, whereby the undesirable deposits referred to above are considerably reduced. Moreover, when using alkaline dyeing, it is in many cases possible to forego reductive or alkaline post-treatment. When employing the acid dye baths which are currently known the dyeing procedure generally represents the sole treatment stage in an acid medium. Accordingly, alkaline dying means that it is possible to avoid a change in pH-value and thus the risk due to entrained alkali can be minimised. A further advantage in this respect is that single-bath dyeing processes of polyester-cellulose fiber mixes using direct dyestuffs also become possible when operating in an alkaline dye bath, and that affords additional options in terms of the choice of suitable direct dyestuffs. Furthermore, when using an alkaline dye bath, it is basically possible to combine the dyeing operation and the operation of washing or desizing polyester materials in one bath. Furthermore recrystallization of fiber decomposition products from splitting and peeling procedures is impeded by the alkali.
However various problems arise in connection with alkaline dyeing, and they can result in unsatisfactory dyeing results, as follows:
On the one hand the hoped-for reduction in oligomer deposits when using alkaline dye baths is frequently not achieved.
Furthermore, the dispersion dyestuffs used in an acid dye bath are in part unstable when used in an alkaline dye bath. In that respect, destruction of the dyestuffs occurs due to the hydrolysis of ester compounds in the side chains of the dyestuff molecule. The degree of such hydrolysis is largely dependent on the pH-value of the dye bath throughout the entire dyeing procedure. In known processes the pH-value is subject to fluctuations which can lead to non-reproducible dyeing results. A further problem is represented by heavy metal ions and polyvalent metal ions in the dye bath, the presence of which cannot be reliably excluded in varying levels of concentration when operating on a large engineering scale. The influence of the heavy metal ions and the metal ions (for example Cu.sup.2+, Fe.sup.2+) on the dyeing result is generally considerably greater when operating with an alkaline dye bath than with an acid dye bath. The influence of constituents such as salts causing hardness in the water of the dye bath on the dyeing result when operating in an alkaline mode is also substantially greater than when operating in an acid mode. Further substances such as for example glucose as a decomposition product of sizing esters can have an adverse influence on the dyeing result in an alkaline procedure. The stronger influence of the above-mentioned substances when operating in an alkaline bath is in part due to the fact that the reduction potential in an alkaline procedure is always greater than in an acid procedure so that irreversible reduction of the dyestuffs can occur.
Furthermore the level of concentration of the adjuvant substances which are dissolved in the dye bath is limited by virtue of the fact that, at an excessively high level of concentration, the dispersion dyestuffs which are generally difficult to dissolve can no longer be held in solution, in spite of the use of dispersing agents.
DE 39 38 631 A1 discloses a process for dyeing polyester-bearing textile material in an alkaline dye bath, which involves adding to the dye bath an amino acid or an amino acid derivative and an alkali metal salt of the amino acid or the amino acid derivative. It has been found however that this process cannot satisfactorily resolve the above-indicated problems.